Gripping means

ABSTRACT

In a gripping means comprising a gripping jaw, or a pair of gripping jaws, mounted in a slot in a support means, said slot having an open mouth into which an object to be gripped can be entered for engagement by the gripping jaw or jaws and opposed plane walls converging towards said open mouth, the or each gripping jaw has an inwardly facing surface comprising inner and outer portions inclined to one another at an obtuse angle and an outwardly facing surface comprising inner and outer portions also inclined to one another at an obtuse angle such that the gripping jaw has the inner portion of its inwardly facing surface inclined to the outer portion of its outwardly facing surface at an angle B which is smaller than the angle C between the outer portion of its inwardly facing surface and the inner portion of its outwardly facing surface and spring means is provided normally urging the outer portion of the inwardly facing surface of the or each gripping jaw in a direction towards the object to be gripped. The gripping means is particularly suitable for use in withdrawing piles from the ground. It may also be used in quickrelease shackles and tensioning devices.

United States Patent Dew [ Feb. 1, 1972 [54] GRIPPING MEANS [72]Inventor: Herbert Oliver Richard Dew, Ross, George Street, Hintlesham,Ipswich, England [22] Filed: Nov. 13, 1970 [21] Appl. No.1 89,386

[52] US. Cl. ..254/29 A, 24/263 SW [51] Int. Cl ..E2lb 19/00 [58] Fieldof Search ..254/29, 29 A; 24/263 R, 263 BW,

24/263 SB, 263 C, 263 HW, 264

Primary Examiner,Andrew R. Juhasz Assistant Examiner-David R. MeltonAttorney-Michael S. Striker [5 7] ABSTRACT In a gripping meanscomprising a gripping jaw, or a pair of gripping jaws, mounted in a slotin a support means, said slot having an open mouth into which an objectto be gripped can he entered for engagement by the gripping jaw or jawsand opposed plane walls converging towards said open mouth, the or eachgripping jaw has an inwardly facing surface comprising inner and outerportions inclined to one another at an obtuse angle and an outwardlyfacing surface comprising inner and outer portions also inclined to oneanother at an obtuse angle such that the gripping jaw has the innerportion of its inwardly facing surface inclined to the outer portion ofits outwardly facing surface at an angle B which is smaller than theangle C between the outer portion of its inwardly facing surface and theinner portion of its outwardly facing surface and spring means isprovided normally urging the outer portion of the inwardly facingsurface of the or each gripping jaw in a direction towards the object tobe gripped.

23 Claims, 8 Drawing Figures PATENTED FEB 11972 3.838.909 sumac? 3 BY 0%L A 7" TQRNE Y Pmmmrw 1m 3.538.909 SHEETZUF 3 BY d/M/ 6% A TTUWWE YWmmrm 1m 3.633.909

sum 30f 3 ATTONEY GRIPPING MEANS This invention relates to a grippingmeans of thekind comprising one gripping jaw or a pair of gripping jawsmounted in a support means and movable into a position to engage anobject to be gripped. In particular, but not exclusively, the inventionrelates to a means for gripping a pile which has been driven into theground, so that by exerting a tractive force on the gripping means thepile can be withdrawn from the ground.

For removing a pile which is embedded in the ground, it is known toemploy a gripping means comprising a support means having an open-endedslot therein. This slot has a pair of opposed walls which are plane andinclined at an acute angle A to one another, these walls convergingtowards each other in the direction away from the bottom ofthe slot.Within the slot there is mounted a pair of gripping jaws of generallywedge shape, the wider ends of the gripping jaws being disposed near tothe bottom of the slot and the outwardly facing and inwardly facingsurfaces of each jaw being inclined to one another at an angleapproximately equal to A/2.

With such a gripping means, it will be appreciated that if thewedge-shaped gripping jaws are moved more deeply into the slot whiletheir outwardly facing surfaces are maintained in contact with saidinclined walls of the slot then the spacing apart of the inwardly facingsurfaces of the gripping jaws will be increased. Conversely, if thegripping jaws are moved in the direction away from the bottom of theslot, their inwardly facing surfaces will be brought closer togetheruntil eventually they abut one another.

In use of the above described gripping means for removing a pile fromthe ground, the support means is lowered onto the upper end portion ofthe pile with the open mouth of the slot facing downwards, so that theupper end portion of the pile enters the slot and is received betweenthe pair of gripping jaws. If an upwardly directed force is now exertedon the support means, for example employing a crane, the gripping jawsmove relatively to the support means in the direction causing theinwardly facing surfaces of the jaws to grip the pile. As the upwardlydirected force on the support means increases, the gripping jaws gripthe pile more and more tightly due to the wedging action between theoutwardly facing surfaces of the gripping jaws and the inclined walls ofthe slot. By a suitable choice of the angle A, the grip exerted on thepile can be sufficiently great for the pile to be withdrawn from theground when the support means is raised. In general, the smaller theangle A is, the greater will be the grip exerted on the pile by .thegripping jaws.

When the pile has been withdrawn, however, it is usually an extremelydifficult and tedious task to release the gripping jaws from the pile,and in general the smaller the angle A is, the greater the difficultybecomes.

Increasing the angle A will facilitate release of the gripping jaws fromthe withdrawn pile, but as the angle A is increased the risk of slippingoccurring between the pile and the gripping jaws during thepile-withdrawing operation is also increased. This increases the timetaken to withdraw a pile and results in damage both to the pile and tothe gripping jaws.

An object of the present invention is to provide an improved grippingmeans of the kind referred to.

According to one aspect of the invention, a gripping means comprising agripping jaw mounted in a slot in a support means said slot having anopen mouth into which an object to be gripped can be entered forengagement by the gripping jaw and opposed plane walls convergingtowards said open mouth, is characterized in that the gripping jaw hasan inwardly facing surface comprising inner and outer portions inclinedto one another at an obtuse angle and an outwardly facing surfacecomprising inner and outer portions also inclined to one another at anobtuse angle such that the gripping jaw has the inner portion of itsinwardly facing surface inclined to the outer portion of its outwardlyfacing surface at an angle B which is smaller than the angle C betweenthe outer portion of its inwardly facing surface and the inner portionof its outwardly facing surface, and spring means normally urging theouter portion of the inwardly facing surface of the gripping jaw in adirection towards the object to be gripped.

In this specification the terms outer" and inner when used in relationto surfaces or ends of the gripping jaw mean, respectively, surfaces orends lying close to or remote from said open mouth of the slot when thegripping jaw is wholly within the slot.

Preferably, there are two gripping jaws in the slot and according tothis aspect of the invention a gripping means comprising a pair ofgripping jaws mounted in a slot in a support means, said slot having anopen mouth into which an object to be gripped can be entered forreception between the gripping jaws and opposed plane walls convergingtowards said open mouth, is characterized in that each gripping jaw hasan inwardly facing surface comprising inner and outer portions inclinedto one another at an obtuse angle and an outwardly facing surfacecomprising inner and outer portions also inclined to one another at anobtuse angle such that each gripping jaw has the inner portion of itsinwardly facing surface inclined to the outer portion of its outwardlyfacing surface at an angle B which is smaller than the angle C betweenthe outer portion of its inwardly facing surface and the inner portionof its outwardly facing surface, and spring means normally urging theouter portions of the inwardly facing surfaces of the gripping jawstowards each other.

The slot is the support means may be open ended. A hole communicatingwith the slot may extend right through the support means with the axisof the hole passing through the open mouth of the slot, so that anobject to be gripped can, if necessary, extend right through the supportmeans.

Preferably, the cross-sectional shape of the gap between the opposedwalls of the slot, in planes perpendicular to the axis of the slot, arerectangular.

The inwardly facing surface of the or each gripping jaw may be linedwith material to improve their frictional grip and/or prevent damage tothe object to be gripped.

Preferably, the spring means comprises a leaf spring and in the case ofa gripping means with two jaws the leaf spring has two limbs each limbbeing interposed between one of said opposed walls of the slot and theinner and outer portions of the outwardly facing surface of a respectivegripping jaw. The leaf spring may consist of a single leaf or aplurality of nested leaves.

When the gripping means according to the invention is used as apile-withdrawing device, the angle B is made equal to or substantiallyequal to half the angle A at which the opposed walls of the slotconverge and the angle A preferably is from 18 to 23. When the angle Ais 18, a suitable value for the angle C is from 1 1 to 14, preferably12.

Various embodiments of gripping means in accordance with the inventionwill now be described, by way of example, with reference to theaccompanying drawings, in which FIG. 1 is a schematic sectionalelevation of a first embodiment of a gripping means intended for use inwithdrawing a pile from the ground,

FIG. 2 is a sectional view taken on the line II-II of FIG. 1, exceptthat the right-hand half of this Figure shows a modified construction ofa support means and a gripping jaw associated therewith, 5

FIG. 3 is a view similar to FIG. 1, but showing the gripping jaws in adifferent position compared with FIG. 1,

FIGS. 4 and 5 are views similar to FIGS. 1 and 2, respectively, of asecond embodiment of a gripping means intended for use in withdrawing apile from the ground, FIG. 5 being a sectional view taken on the lineV-V of FIG. 4 except that the right-hand half of FIG. 5 shows a modifiedconstruction of a support means and a gripping jaw associated therewith,

FIG. 6 is a schematic sectional elevation of a third embodiment of agripping means intended for use in withdrawing a pile from the ground,

FIG. 7 is a schematic sectional elevation of a quick-release shackleincorporating a gripping means, and

FIG. 8 is a schematic sectional elevation of a tensioning deviceincorporating a gripping means.

The gripping means shown in FIG. 1 and the left-hand half of FIG. 2comprises a support means 1, for example a forged steel block, having anopen-ended slot 2 formed therein. The slot 2 has opposed, accuratelymachined plane walls 3, 4 which are inclined to one another at an angleA and converge towards the open mouth 5 of the slot. The walls 3, 4 aresymmetrically disposed with respect to a plane WW disposed at rightangles to the plane of the paper and at right angles to the planesurface 6 ofthe support means 1. Furthermore, the walls 3, 4 aredisposed so that the cross-sectional shape of the gap between the walls,in planes perpendicular to the axis of the slot, is rectangular. Thesupport means 1 is provided with any suitable means (not shown) forexample an apertured lug or shackle, by means of which it can be liftedby a hoisting device, for example a crane.

Within the slot 2 are two identical gripping jaws 7, 8 of double wedgeshape. The outwardly facing surface of each of these jaws has a recess16 formed therein, the bottom of the recess being formed by a planeouter portion 9 and a plane inner portion 10, the plane portion 9 beinginclined to the plane portion 10 at an obtuse angle D. The inwardlyfacing surface of each of the jaws 7, 8 has a plane outer portion 11 anda plane inner portion 12, these two portions being inclined to oneanother at an obtuse angle E. The portions 9 and 10 of the outwardlyfacing surfaces and the portions 11 and 12 of the inwardly facingsurfaces of the jaws are accurately machined. The angles D and E arechosen such that the acute angle B between the surface portions 9 and 12is equal to A/2 and the acute angle C between the surface portions 10and II is greater than A/2. Furthermore, the lengths of the portions9-12 are chosen such that the vertex F of the angle D is closer to thesurface 6 than the vertex G of the angle E.

A leaf spring 13 bent into substantially U-shape is mounted in the slot2 with its limb l4 interposed between the jaw 7 and the wall 3 and itslimb 15 (shown partly broken away in FIGS. 1 and 2) interposed betweenthe jaw 8 and the wall 4, these limbs being received in the recesses 16formed in the outwardly facing surfaces of the jaws. Although the spring30 is shown as having a single leaf, it will be appreciated that it canhave a plurality of rested leaves.

FIG. 1 shows the situation when the support means I is lowered on to apile 17 which it will be assumed has its lower end portion (not shown)embedded in the ground. The upper end portion of the pile enters theslot 2 between the gripping jaws 7, 8 and the latter are held by thespring 13 in the position in which the outer portions 11 of the inwardlyfacing surfaces of the jaws engage the pile. If now the support means 1is raised by a force acting in the direction of the arrow X the jawsrotate against the action of the spring limbs l4, l5 and the innerportions I2 of the inwardly facing surfaces of the jaws grip the pileand the outer portions 9 of the outwardly facing surfaces of the jawsforce the spring limbs l4, 15 against the walls 3, 4, respectively ofthe slot 2, see FIG. 3. The effective wedge angle of the jaws 7, Sis nowthe angle A/2 and continued upward movement of the support means 1results in the gripped pile being withdrawn from the ground.

When the pile 17 has been withdrawn from the ground, the spring 14reasserts itself and rocks the jaws 7, 8 back to their positions shownin FIG. 1. The effective wedge angle of the jaws is now greater than theangle A/2, so facilitating release of the pile from the gripping means.

In the embodiment shown in the drawing the angle A is equal to 18, theangle B is 9 and the angle C is 12.

The right-hand portion of FIG. 2 shows a modified form of support means1a and an associated jaw 8a of modified crosssectional shape. The jaw 8adiffers from the jaw 8 in that it has outwardly facing flanges 18 whichare received in relieved portions 19 of the support means la to preventlateral movement of the jaw in the slot 2. The jaw 7 would, of course,be similarly modified.

In the gripping means of FIGS. 1 to 3, the vertex F of the angle D isalways closer to the surface 6 then the vertex G of the angle E. This isessential in the case ofa gripping means in which easy return of thejawsfrom the position of FIG. 3 to the position of Flg. l is required afteruse of the gripping means, for example when the gripping means isemployed for withdrawing piles. In some applications of the grippingmeans according to the invention, however, an overcenter gripping actionis desirable. This is achieved in the gripping means of FIGS. 1 to 3 byshaping the jaws so that in the FIG. 1 position the vertex F is closerto the surface 6 than the vertex G, but that in the FIG. 3 position thevertex G is closer to the surface 6 than the vertex F. To enable this tooccur, a recess is provided in each of the walls 3 and 4, these recessesbeing so dimensioned as to allow local deflection 0f the spring limbs14, 15 intermediate their ends. A suitable recess for this purpose isshown in dotted lines in the wall 3 in FIG. 1, this recess beingdesignated by the numeral 20.

The gripping means shown in FIGS. 4 and 5 are similar to those shown inFIGS. 1-3, except that the leaf spring 13 of FIG. 1-3 is replaced by apair of springs 13a, 13b made from steel rod of circular cross section.These springs 13a, 13b are located in recesses 21 in the jaws 7, 8 (or8a). It will be appreciated that in this embodiment of the grippingmeans the outwardly facing plane surface portions 9a and 10a of the jaws7, 8 (or 8a) make direct contact with the slot walls 3, 4, respectively.These embodiments of the gripping means are suitable for applicationswhere metal seizure between the jaws and the walls of the slot is highlyimprobable.

The gripping means described above with reference to FIGS. 1 to 5 allhave two gripping jaws, but it will be appreciated that one of the jawsmay be omitted to provide a gripping means with a single jaw. Oneembodiment of such a gripping means is shown in FIG. 6 and is a modifiedform of the gripping means of FIGS. 1 to 3. Parts of the gripping meansof FIG. 6 which are similar to those of the gripping means of FIGS. 1 to3 have been designated by the same reference numerals with the additionofa prime. In FIG. 6 the support means 1 has an open-ended slot 2 formedtherein. The slot 2 has opposed, accurately machined plane walls 3, 4which are inclined to one another at an angle A and converge towards theopen mouth 5 of the slot. The wall 4' is disposed parallel to a plane WWdisposed at right angles to the plane of the paper and at right anglesto the plane surface 6 of the support means 1. Furthermore, the walls 3,4 are disposed so that the cross-sectional shape of the gap between thewalls, in planes, perpendicular to the axis of the slot, is rectangular.The support means 1 is provided with any suitable means (not shown) forexample an apertured lug or shackle, by means of which it can be liftedby a hoisting device, for example a crane.

Within the slot 2 is a gripping jaw 7 of double wedge shape. Theoutwardly facing surface of the jaw 7 has a recess 16' formed therein,the bottom of the recess being formed by a plane outer portion 9' and aplane inner portion I0, the plane portion 9 being inclined to the planeportion 10 at an obtuse angle D. The inwardly facing surface of the jaw7 has a plane outer portion 11' and a plane inner portion 12, these twoportions being inclined to one another at an obtuse angle E. Theportions 9 and 10 of the outwardly facing surface and the portions 11'and 12 of the inwardly facing surface of the jaw are accuratelymachined. The angles D and E are chosen such that the acute angle Bbetween the surface portions 9 and 12 is equal to A and the acute angleC between the surface portions 10 and 11 is greater than A. Furthermore,the lengths of the portions 9-12 are chosen such that the vertex F ofthe angle D is closer to the surface 6' than the vertex G of the angleE.

A leaf spring 13 bent into substantially U-shape with limbs 14 and 15 ismounted in the slot 2' with its limb 14 (shown partly broken away inFIGS. 1 and 2) interposed between the jaw 7 and the wall 3 this limbbeing received in the recess 16.

The manner of operation of the gripping means of FIG. 6 is similar tothat of FIGS. 1 to 3.

The gripping means described above with reference to FIGS. 1-6 areintended primarily for withdrawing piles from the ground. It will,however, be appreciated that the same gripping means can be employed forlifting objects and transporting them from one place to another. When itis intended to use the gripping means for lifting fragile or easilydamaged objects, for example sheets of glass, it is desirable to linethe inwardly facing surfaces of the jaws 7, 8 (or 8a) with a softmaterial, for example rubber.

FIG. 7 illustrates the employment of the gripping means of FIGS. 4 and 5in a quick-release shackle intended for pitching sheet piles intoposition preparatory to pile driving. Some existing quick-releaseshackles for this purpose require a hole to be drilled in each pileadjacent to the end which is uppermost during the pile-drivingoperation, the shackle being engaged with the pile using this hole. Thequick-release shackle shown in FIG. 7 avoids the necessity for drillingeach pile.

The quick-release shackle shown in FIG. 7 includes a support means lb,jaws 7, 8 and springs 13b similar to either of the gripping means ofFIGS. 4 and 5. A hole 25 in the support means lb communicates with theslot 2 and houses a block 26 provided with ratchet teeth 27 on itsexternal surface. A spring-loaded detent 28 slidably mounted in thesupport means lb engages the ratchet teeth 27. A screw-threaded rod 29is threadedly engaged in a bore 30 passing through the block 26, theaxes of the bore 30 and the hole 25 being substantially coincident anddisposed substantially in the plane of symmetry W-W of the slot 2.Externally of the support means 1b the rod 30 is provided with ahandwheel 31. Inside the slot 2, the rod 30 is connected to a member 32provided with projections 33, 34 which engage the inner ends of the jaws7, 8 respectively. The support means lb is provided with an arm 35 bymeans of which the support means can be connected to a hoisting device,for example to the hook of a crane.

In use of the quick-release shackle shown in FIG. 7, the jaws 7, 8 areengaged with the end portion of a pile l7 and the jaws are then moved inthe direction of the arrow Y, by the rotation of the handwheel 31, so asto grip the pile. The ratchet and detent mechanism 27, 28 allows theshackle to accommodate piles of different thickness. When the jaws 7, 8are gripping -'the pile, the support means is lifted by means of ahoisting device (not shown) connected to the arm 35, and the pile ispitched vertically into the desired position. Any fouling of the pile onother objects, or any upward thrust exerted on the pile, during thispitching procedure does not cause any slackening of the grip of the jaws7, 8 on the pile, since the member 32 acts as a stop means to limitmovement of the jaws 7, 8 into the slot 2. When the pile is pitched inthe desired position the shackle is released by withdrawing the detent28 from the ratchet teeth 27 and then lowering the shackle, so enablingthe jaws to rock on their vertices F and then moves more deeply into theslot 2 and thus release their grip on the pile.

It will of course, be appreciated that the gripping means of FIGS. 1-3may be used, instead of the gripping means of FIGS. 4 and 5, in thequick-release shackle of FIG. 7, the member 32 and/or the spring 13being suitably shaped so that they do not interfere with each other.

FIG. 8, illustrates a tensioning device which includes the grippingmeans of FIGs. l-3. In this device a support means 10, which houses thejaws 7, 8 and the leaf spring 13, is slidably mounted in a casing 40 andat its end remote from its surface 6 the support means is provided witha screw-threaded adjustment means in the form of screw-threaded rod 41.The axis of the rod 41 lies substantially, in the plane of symmetry -Wof the slot 2 and the support member is slidable in the casing 40 in adirection parallel to the axis of the rod 41. The rod 41 passes througha boss 42 on an end cover 43 of the casing 40 and receives a nut 44externally of the casing. Between the nut 44 and the boss 42 there isarranged a thrust bearing 45 surrounding the rod 41. A handle 46 isprovided on the nut 44 for the purpose of facilitating turning of thenut. An indicating device 47, shown purely schematically, is connectedto the free ends ofthe limbs l4, l5 ofthe spring 13 for the purpose ofindicating the separation of the ends of these limbs.

In FIG. 8 the tensioning device is shown employed in tensioning a rod48, for example a reinforcing rod in a concrete object 49. The rod 48has a screw-threaded portion 50 which receives a nut 51. A washer 52 isinterposed between the nut 51 and the object 49. Adjacent to thescrew-threaded portion 50, the rod 48 has an axially extending integralend portion 53.

In use of the tensioning device shown in FIG. 8, the portion 53 of therod 48 is introduced between the jaws 7, 8 and a flanged end 54 of thecasing is applied against the object 49. The support means 1c is thenmoved in the direction away from the object 49, by rotation of the nut44. During the initial stages of rotation of the nut 44 the supportmeans 10 moves relative to the spring 13 so that the inner ends of thespring limbs engage against the walls of the slot 2. At the same timethe jaws 7, 8 grip the rod portion 53 and apply a tensile force to therod 48. As rotation of the nut 44 is continued, the jaws 7, 8 graduallyforce the spring limbs 14, 15 apart as the tensile force exerted on therod 48 increases. The indicating device 47, which measures theseparation of the free ends of the spring limbs 14, 15 is calibrated interms of the load exerted on the rod 48. When the indicating device 47shows that rod 48 has been tensioned to the desired degree, the nut 51is tightened against the object 49 and then the tensioning devicesreleased by turning the nut 44 in the direction allowing the supportmeans 10 to approach the object 49. It will be appreciated that themaximum load indicated by the device 47 is achieved before the outerends of the springs 14, 15 bear against the walls of the slot 2.

What is claimed is:

1. A gripping means comprising a gripping jaw mounted in a slot in asupport means, said slot having an open mouth into which an object to begripped can be entered for engagement by the gripping jaw and opposedplane walls converging towards said open mouth, characterized in thatthe gripping jaw has an inwardly facing surface comprising inner andouter portions inclined to one another at an obtuse angle and anoutwardly facing surface comprising inner and outer portions alsoinclined to one another at an obtuse angle such that the gripping jawhas the inner portion of its inwardly facing surface inclined to theouter portion of its outwardly facing surface at an angle B which issmaller than the angle C between the outer portion of its inwardlyfacing surface and the inner portion of its outwardly facing surface andin that spring means is provided normally urging the outer portion ofthe inwardly facing surface of the gripping jaw in a direction towardsthe object to be gripped.

2. A gripping means comprising a pair of gripping jaws mounted in a slotin a support means, said slot having an open mouth into which an objectto be gripped can be entered for reception between the gripping jaws andopposed plane walls converging towards said open mouth, characterized inthat each gripping jaw has an inwardly facing surface comprising innerand outer portions inclined to one another at an obtuse angle and anoutwardly facing surface comprising inner and outer portions alsoinclined to one another at an obtuse angle such that each gripping jawhas the inner portion of its inwardly facing surface inclined to theouter portion of its outwardly facing surface at an angle B which issmaller than the angle C between the outer portion of its inwardlyfacing surface and the inner portion of its outwardly facing surface,and spring means normally urging the outer portions of the inwardlyfacing surfaces of the gripping jaws towards each other.

3. A gripping means according to claim 2, in which the cross-sectionalshape the gap between the walls, in planes perpendicular to the axis ofthe slot is rectangular.

4. A gripping means according to claim 3, in which the inner and outerportions of the outwardly facing surface of the or each gripping jawhave plane surface portions.

5. A gripping means according to claim 2, in which the spring meanscomprises a leaf spring having two limbs, each limb being interposedbetween one of said opposed walls of the slot and the inner and outerportions of the outwardly facing surface ofa respective grippingjaw.

6. A gripping means according to claim 5, in which the leaf springconsists ofa single leaf or a plurality of nested leaves.

7. A gripping means according to claim 2, in which a hole communicatingwith the slot extends right through the support means, the axis of thehole passing through the open mouth of the slot.

8. A gripping means according to claim 2, in which the angle B is madeequal to or substantially equal to half the angle A at which saidopposed walls of the slot converge.

9. A gripping means according to claim 8, in which the angle A is from18 to 23.

10. A gripping means according to claim 9, in which the angle A is l8,the angle B is 9 and the angle C is from 1 1 to l4.

11. A gripping means according to claim 10, in which the angle C is 12.

12. A pile-withdrawing means comprising a gripping jaw mounted in a slotin a support means, said slot having an open mouth into which a pile tobe withdrawn from the ground can be entered for engagement by thegripping jaw and opposed plane walls converging towards said open mouth,characterized in that the gripping jaw has an inwardly facing surfacecomprising inner and outer portions inclined to one another at an obtuseangle and an outwardly facing surface comprising inner and outerportions also inclined to one another at an obtuse angle such that thegripping jaw has the inner portion of its inwardly facing surfaceinclined to the outer portion of its outwardly facing surface at anangle B which is smaller than the angle C between the outer portion ofits inwardly facing surface and the inner portion of its outwardlyfacing surface and in that spring means is provided normally urging theouter portion of the inwardly facing surface of the gripping jaw in adirection towards the pile to be withdrawn.

13. A pile-withdrawing means comprising a pair of gripping jaws mountedin a slot in a support means, said slot having an open mouth into whicha pile to be withdrawn from the ground can be entered for receptionbetween the gripping jaws and opposed plane walls converging towardssaid open mouth, characterized in that each gripping jaw has an inwardlyfacing surface comprising inner and outer portions inclined to oneanother at an obtuse angle and an outwardly facing surface comprisinginner and outer portions also inclined to one another at an obtuse anglesuch that each gripping jaw has the inner portion of its inwardly facingsurface inclined to the outer portion of its outwardly facing surface atan angle B which is smaller than the angle C between the outer portionof its inwardly facing surface and the inner portion of its outwardlyfacing surface, and spring means normally urging the outer portions ofthe inwardly facing surfaces of the grippingjaws towards each other.

14. A pile-withdrawing device according to claim .13, in which saidsupport means is provided with an apertured lug or shackle by means ofwhich the support means can be lifted by a hoisting device.

15. A uick-release shackle comprising a gripping jaw mounted in a slotin a support means, said slot having an open mouth into which an objectto be hoisted can be entered for engagement by the gripping jaw andopposed plane walls converging towards said open mouth, and means on thesupport means enabling connection of the support means to a hoistingdevice, characterized in that the gripping jaw has an inwardly facingsurface comprising inner and outer portions inclined to one another atan obtuse angle and an outwardly facing surface comprising inner andouter portions also inclined to one another at an obtuse angle such thatthe gripping jaw has the inner portion of its inwardly facing surfaceinclined to the outer portion of its outwardly facing surface at anangle B which is smaller than the angle C between the outer portion ofits inwardly facing surface and the inner portion of its outwardlyfacing surface and in that spring means is provided normally urging theouter portion of the inwardly facing surface of the gripping jaw in adirection towards the object to be hoisted.

16. A quick-release shackle comprising a pair of gripping jaws mountedin a slot in a support means, said slot having an open mouth into whichan object to be hoisted can be entered for reception between thegripping jaws and opposed plane walls converging towards said openmouth, and means on the support means enabling connection of the supportmeans to a hoisting device, characterized in that each gripping jaw hasan inwardly facing surface comprising inner and outer portions inclinedto one another at an obtuse angle and an outwardly facing surfacecomprising inner and outer portions also inclined to one another at anobtuse angle such that each gripping jaw has the inner portions ofitsinwardly facing surface inclined to the outer portion of its outwardlyfacing surface at an angle B which is smaller than the angle C betweenthe outer portion of its inwardly facing surface and the inner portionof its outwardly facing surface, and spring means normally urging theouter portions of the inwardly facing surfaces of the grippingjawstowards each other.

17. A quick-release shackle according to claim 16, comprising anadjustable stop means carried by the support means, which stop meanslimits the movement of the jaws in the direction away from said openmouth of the slot.

18. A quick-release shackle according to claim 17, in which saidadjustable stop means is releasably carried in the support means.

19. A quick-release shackle according to claim 18, in which theadjustable stop means is releasably carried in the support means by adetent and ratchet mechanism.

20. A tensioning device comprising a casing, a support means slidablymounted in the casing, a gripping jaw mounted in a slot in said supportmeans, said slot having an open mouth into which an object to betensioned can be entered for engagement by the gripping jaw and opposedplane walls converging towards said open mouth, and means for adjustingthe position of the support means in the casing, characterized in thatthe gripping jaw has an inwardly facing surface comprising inner andouter portions inclined to one another at an obtuse angle and anoutwardly facing surface comprising inner and outer portions alsoinclined to one another at an obtuse angle such that the gripping jawhas the inner portion of its inwardly facing surface inclined to theouter portion of its outwardly facing surface at an angle B which issmaller than the angle C between the outer portion of its inwardlyfacing surface and the inner portion of its outwardly facing surface andin that spring means is provided normally urging the outer portion ofthe inwardly facing surface of the gripping jaw in a direction towardsthe object to be tensioned.

21. A tensioning device comprising a casing, a support means slidablymounted in the casing, a pair of gripping jaws mounted in a slot in asupport means, said slot having an open mouth into which an object to betensioned can be entered for reception between the gripping jaws andopposed plane walls converging towards said open mouth, and means foradjusting the position of the support means in the casing, characterizedin that each gripping jaw has an inwardly facing surface comprisinginner and outer portions inclined to one another at an obtuse angle andan outwardly facing surface comprising inner and outer portions alsoinclined to one another at an obtuse angle such that each gripping jawhas the inner portion of its inwardly facing surface inclined to theouter portion of its outwardly facing surface at an angle B which issmaller than the angle C between the outer portion of its inwardlyfacing surface and the inner portion of its outwardly facing surface,and spring means normally urging the outer portions of the inwardlyfacing surfaces of the gripping jaws towards each other.

22. A tensioning device according to claim 21, comprising screw-threadedadjustment means for adjusting the position of the support means in thecasing.

23. A tensioning device accordin g to claim 21, comprising a

1. A gripping means comprising a gripping jaw mounted in a slot in asupport means, said slot having an open mouth into which an object to begripped can be entered for engagement by the gripping jaw and opposedplane walls converging towards said open mouth, characterized in thatthe gripping jaw has an inwardly facing surface comprising inner andouter portions inclined to one another at an obtuse angle and anoutwardly facing surface comprising inner and outer portions alsoinclined to one another at an obtuse angle such that the gripping jawhas the inner portion of its inwardly facing surface inclined to theouter portion of its outwardly facing surface at an angle B which issmaller than the angle C between the outer portion of its inwardlyfacing surface and the inner portion of its outwardly facing surface andin that spring means is provided normally urging the outer portion ofthe inwardly facing surface of the gripping jaw in a direction towardsthe object to be gripped.
 2. A gripping means comprising a pair ofgripping jaws mounted in a slot in a support means, said slot having anopen mouth into which an object to be gripped can be entered forreception between the gripping jaws and opposed plane walls convergingtowards said open mouth, characterized in that each gripping jaw has aninwardly facing surface comprising inner and outer portions inclined toone another at an obtuse angle and an outwardly facing surfacecomprising inner and outer portions also inclined to one another at anobtuse angle such that each gripping jaw has the inner portion of itsinwardly facing surface inclined to the outer portion of its outwardlyfacing surface at an angle B which is smaller than the angle C betweenthe outer portion of its inwardly facing surface and the inner portionof its outwardly facing surface, and spring means normally urging theouter portions of the inwardly facing surfaces of the gripping jawstowards each other.
 3. A gripping means according to claim 2, in whichthe cross-sectional shape the gap between the walls, in planesperpendicular to the axis of the slot is rectangular.
 4. A grippingmeans according to claim 3, in which the inner and outer portions of theoutwardly facing surface of the or each gripping jaw have plane surfaceportions.
 5. A gripping means according to claim 2, in which the springmeans comprises a leaf spring having two Limbs, each limb beinginterposed between one of said opposed walls of the slot and the innerand outer portions of the outwardly facing surface of a respectivegripping jaw.
 6. A gripping means according to claim 5, in which theleaf spring consists of a single leaf or a plurality of nested leaves.7. A gripping means according to claim 2, in which a hole communicatingwith the slot extends right through the support means, the axis of thehole passing through the open mouth of the slot.
 8. A gripping meansaccording to claim 2, in which the angle B is made equal to orsubstantially equal to half the angle A at which said opposed walls ofthe slot converge.
 9. A gripping means according to claim 8, in whichthe angle A is from 18* to 23* .
 10. A gripping means according to claim9, in which the angle A is 18*, the angle B is 9* and the angle C isfrom 11* to 14* .
 11. A gripping means according to claim 10, in whichthe angle C is 12* .
 12. A pile-withdrawing means comprising a grippingjaw mounted in a slot in a support means, said slot having an open mouthinto which a pile to be withdrawn from the ground can be entered forengagement by the gripping jaw and opposed plane walls convergingtowards said open mouth, characterized in that the gripping jaw has aninwardly facing surface comprising inner and outer portions inclined toone another at an obtuse angle and an outwardly facing surfacecomprising inner and outer portions also inclined to one another at anobtuse angle such that the gripping jaw has the inner portion of itsinwardly facing surface inclined to the outer portion of its outwardlyfacing surface at an angle B which is smaller than the angle C betweenthe outer portion of its inwardly facing surface and the inner portionof its outwardly facing surface and in that spring means is providednormally urging the outer portion of the inwardly facing surface of thegripping jaw in a direction towards the pile to be withdrawn.
 13. Apile-withdrawing means comprising a pair of gripping jaws mounted in aslot in a support means, said slot having an open mouth into which apile to be withdrawn from the ground can be entered for receptionbetween the gripping jaws and opposed plane walls converging towardssaid open mouth, characterized in that each gripping jaw has an inwardlyfacing surface comprising inner and outer portions inclined to oneanother at an obtuse angle and an outwardly facing surface comprisinginner and outer portions also inclined to one another at an obtuse anglesuch that each gripping jaw has the inner portion of its inwardly facingsurface inclined to the outer portion of its outwardly facing surface atan angle B which is smaller than the angle C between the outer portionof its inwardly facing surface and the inner portion of its outwardlyfacing surface, and spring means normally urging the outer portions ofthe inwardly facing surfaces of the gripping jaws towards each other.14. A pile-withdrawing device according to claim 13, in which saidsupport means is provided with an apertured lug or shackle by means ofwhich the support means can be lifted by a hoisting device.
 15. Aquick-release shackle comprising a gripping jaw mounted in a slot in asupport means, said slot having an open mouth into which an object to behoisted can be entered for engagement by the gripping jaw and opposedplane walls converging towards said open mouth, and means on the supportmeans enabling connection of the support means to a hoisting device,characterized in that the gripping jaw has an inwardly facing surfacecomprising inner and outer portions inclined to one another at an obtuseangle and an outwardly facing surface comprising inner and outerportions also inclined to one another at an obtuse angle such that thegripping jaw has the inner portion of its inwardly facing surfaceinclined to the outer portion of its outwardly facing surface at anangle B which is smaller than the angle C between the outer portion ofits inwardly facing surface and the inner portion of its outwardlyfacing surface and in that spring means is provided normally urging theouter portion of the inwardly facing surface of the gripping jaw in adirection towards the object to be hoisted.
 16. A quick-release shacklecomprising a pair of gripping jaws mounted in a slot in a support means,said slot having an open mouth into which an object to be hoisted can beentered for reception between the gripping jaws and opposed plane wallsconverging towards said open mouth, and means on the support meansenabling connection of the support means to a hoisting device,characterized in that each gripping jaw has an inwardly facing surfacecomprising inner and outer portions inclined to one another at an obtuseangle and an outwardly facing surface comprising inner and outerportions also inclined to one another at an obtuse angle such that eachgripping jaw has the inner portions of its inwardly facing surfaceinclined to the outer portion of its outwardly facing surface at anangle B which is smaller than the angle C between the outer portion ofits inwardly facing surface and the inner portion of its outwardlyfacing surface, and spring means normally urging the outer portions ofthe inwardly facing surfaces of the gripping jaws towards each other.17. A quick-release shackle according to claim 16, comprising anadjustable stop means carried by the support means, which stop meanslimits the movement of the jaws in the direction away from said openmouth of the slot.
 18. A quick-release shackle according to claim 17, inwhich said adjustable stop means is releasably carried in the supportmeans.
 19. A quick-release shackle according to claim 18, in which theadjustable stop means is releasably carried in the support means by adetent and ratchet mechanism.
 20. A tensioning device comprising acasing, a support means slidably mounted in the casing, a gripping jawmounted in a slot in said support means, said slot having an open mouthinto which an object to be tensioned can be entered for engagement bythe gripping jaw and opposed plane walls converging towards said openmouth, and means for adjusting the position of the support means in thecasing, characterized in that the gripping jaw has an inwardly facingsurface comprising inner and outer portions inclined to one another atan obtuse angle and an outwardly facing surface comprising inner andouter portions also inclined to one another at an obtuse angle such thatthe gripping jaw has the inner portion of its inwardly facing surfaceinclined to the outer portion of its outwardly facing surface at anangle B which is smaller than the angle C between the outer portion ofits inwardly facing surface and the inner portion of its outwardlyfacing surface and in that spring means is provided normally urging theouter portion of the inwardly facing surface of the gripping jaw in adirection towards the object to be tensioned.
 21. A tensioning devicecomprising a casing, a support means slidably mounted in the casing, apair of gripping jaws mounted in a slot in a support means, said slothaving an open mouth into which an object to be tensioned can be enteredfor reception between the gripping jaws and opposed plane wallsconverging towards said open mouth, and means for adjusting the positionof the support means in the casing, characterized in that each grippingjaw has an inwardly facing surface comprising inner and outer portionsinclined to one another at an obtuse angle and an outwardly facingsurface comprising inner and outer portions also inclined to one anotherat an obtuse angle such that each gripping jaw has the inner portion ofits inwardly facing surface inclined to the outer portion of itsoutwardly facing surface at an angle B which is smaller than the angle Cbetween the outer portion of its inwardly facing surface and the innerportion of its outwardly facing surface, and spring means normallyurging the outer portions of the inwardly facing surfaces of thegripping jaws towards each other.
 22. A tensioning device according toclaim 21, comprising screw-threaded adjustment means for adjusting theposition of the support means in the casing.
 23. A tensioning deviceaccording to claim 21, comprising a tension-indicating device.